Cloud manager login method and SSO login note

[m-sulik]

This PR adds information about the using Akamai Control Center SSO to log in to Cloud Manager.

• Accounts > Security Controls for User Accounts: Added the Single sign-on (SSO) with Akamai Control Center section, with details on this feature.
• Platform > Getting Started on the Linode Platform: Added a note about the ACC SSO feature with a link to the Security Controls page.
• Accounts > Enable Third Party Authentication on Your User Account: Added a note about the ACC SSO feature with a link to the Security Controls page. Also updated the Linode login method to Cloud Manager.

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